We start newbies training with multiple attackers pretty early (almost as soon as they can roll decently). For beginning nage, the ukes are limited to one or two kinds of strike, nage gets at most two different throws to try, and things are deliberately kept half-speed or less. It's more of a stretching exercise for your mind. It helps make you aware of your surroundings, where your throw is going, and reminds you that other things can happen while you're dealing with something.